J.F. Connolly

A positron camera for industrial application

Abstract A positron camera for application to flow tracing and measurement in mechanical subjects is described. It is based on two 300 × 600 mm 2 hybrid multiwire detectors; the cathodes are in the form of lead strips plated onto printed-circuit board, and delay lines are used to determine the location of photon interactions. Measurements of the positron detection efficiency (30 Hz μCi −1 for a centred unshielded source), the maximum data logging rate (3 kHz) and the spatial resolving power (point source response = 5.7 mmfwhm) are presented and discussed, and results from initial demonstration experiments are shown.

The Rutherford Appleton laboratory's mark I multiwire proportional counter positron camera

Abstract A small (30 cm × 30 cm) model of a proposed large aperture positron camera has been developed at Rutherford Appleton Laboratory. Based on multiwire proportional counter technology it uses lead foil cathodes which function simultaneously as converters for the 511 keV gamma rays and readout electrodes for a delay line readout system. The detectors have been built up into a portable imaging system complete with a dedicated computer for data taking, processing and display. This has permitted evaluation of this type of positron imaging system in the clinical environment using both cyclotron generated isotopes ( 15 O, 11 C, 18 F, 124 I) and available isotopic generator systems ( 82 Rb, 68 Ga). At RAL we provided a complete hardware system and sufficient software to permit our hospital based colleagues to generate useful images with the minimum of effort. A complete description of the system is given with performance figures and some of the images obtained in three hospital visits are presented. Some detailed studies of the imaging performance of the positron camera are reported which have bearing on the design of future, improved systems.

Two digital X-ray imaging systems for applications in X-ray diffraction

Abstract Two digital X-ray imaging systems developed at the Rutherford Appleton Laboratory are described: the Mark I and the Mark II. Both use a bidimensionally sensitive multiwire proportional counter (MWPC) as the basic X-ray image transducer coupled, in the case of the Mark I to a Digital LSI 11–23 microcomputer system via CAMAC, and in the case of the Mark II to a Digital LSI 11–73 microcomputer system via custom-built data acquisition hardware mounted directly on the Q-bus of the microcomputer. The Mark I system provides the advantages of high speed, high sensitivity digital imaging directly into the computer with the potential for software control of the sample orientation and environment. The Mark II system adds the novel features of signal averaging and multiframe exposures. The dedicated digital memories have a resolution of 512×512 pixels of 16 bits, matching well to the spatial resolution of the xenon-filled MWPC (0.5 mm fwhm over an aperture of 200 mm×200 mm). A 512×512×4 bit video graphics system displays the images in grey scales or colour.

High speed quantitative digital beta autoradiography using a multistep avalanche detector and an Apple II microcomputer

Abstract The development of an electronic, digital beta autoradiography system is described. Using a multistep avalanche/multiwire proportional counter (MSA/MWPC) detector system fitted with delay line readout, high speed digital imaging is demonstrated with submillimeter spatial resolution. In the case of autoradiography with a tritium label, image acquisition requires about one hour compared with several weeks for conventional film techniques. Good proportionality of observed counting rate relative to the known tritium activity is demonstrated. The application of the system to autoradiography in immunoelectrophoresis, histopathology and DNA sequencing is described (using 125I, 14C and 35S labels in addition to 3H).

The development of gas microstrip detectors for high energy physics applications

Abstract Results are presented from the on-going programme of work at Rutherford Appleton Laboratory aimed at the development of high spatial resolution, high rate tracking counters for the next generation of collider experiments.

A gas microstrip wide angle X-ray detector for application in synchrotron radiation experiments

Abstract The Gas Microstrip Detector has counting rate capabilities several orders of magnitude higher than conventional wire proportional counters while providing the same (or better) energy resolution for X-rays. In addition the geometric flexibility provided by the lithographic process combined with the self-supporting properties of the substrate offers many exciting possibilities for X-ray detectors, particularly for the demanding experiments carried out on Synchrotron Radiation Sources. Using experience obtained in designing detectors for Particle Physics we have developed a detector for Wide Angle X-ray Scattering studies. The detector has a fan geometry which makes possible a gas detector with high detection efficiency, sub-millimetre spatial resolution and good energy resolution over a wide range of X-ray energy. The detector is described together with results of experiments carried out at the Daresbury Laboratory Synchrotron Radiation Source.

Studies of Lead Tungstate Crystal Matrices in High-Energy Beams for the CMS Electromagnetic Calorimeter at the LHC

Using matrices of lead tungstate crystals, energy resolutions better than 0.6% at 100 GeV have been achieved in the test beam in 1995. It has been demonstrated that a lead tungstate electromagnetic calorimeter read out by avalanche photodiodes can consistently achieve the excellent energy resolutions necessary to justify its construction in the CMS detector. The performance achieved has been understood in terms of the properties of the crystals and photodetectors.

Performance characterisation of large area BaF2-TMAE detectors for use in a whole body clinical PET camera

The first of two large area detectors which form part of a new whole body positron scanner has been fully characterised and a performance assessment has been completed. The detector consists of an array of crystals coupled to a large area, low pressure multiwire chamber filled with tetrakis-(dimethylamino)-ethylene (TMAE) gas. A complete optimisation has been carried out in the operation of the chamber in order to establish optimal levels of stability and performance. Under the optimised operational conditions the detector exhibits an efficiency of ∼ 25%, with a time and position resolution of 3.5 ns (FWHM) and 6.5 mm (FWHM) respectively. A Monte Carlo simulation of the complete dual headed PET camera has been used to assess the expected image quality in terms of contrast and signal to noise ratio. The simulation has been also used to establish the effect of utilising crystal slabs and a support frame to cover the active area of each detector in comparison with a single sheet of BaF2. The results demonstrate that the use of crystal tiles results in a small increase in the background noise, not significantly compromising the image quality.

Results from tests on matrices of lead tungstate crystals using high energy beams

The performance of lead tungstate crystals using photomultipliers and Si avalanche photodiodes to detect the scintillation light has been studied using high energy electron, pion and muon beams at CERN. Results from tests carried out in 1993 and 1994 are presented. Good energy resolution has been obtained using photomultipliers. Some further development is required of crystals and avalanche photodiodes in order to achieve a performance, with avalanche photodiode readout, similar to that obtained using photomultipliers.

Development studies for a high-rate positron camera based on a BaF2/TMAE system

Abstract Clinical experience with our existing (Mark-II) positron camera has shown that the modest sensitivity and low maximum data rate of the device limit its clinical application. The latter problem is caused by the high singles rates encountered by the detectors and the sensitivity is limited by the conversion efficiency of the 511 keV gamma rays. We report the progress of our studies into a new positron camera system based on barium fluoride scintillators coupled to a low-pressure TMAE-filled MWPC which is read out by fast delay lines. According to our estimates this system will yield a sensitivity and a maximum data rate more than an order of magnitude higher than our existing system with comparable spatial resolution.